Communications networks use a transmission medium to transmit information in the form of computer data, voice, music, video, etc., from one station to another. The communications medium may be a wired link, a fiber optic link, or a wireless link. The wireless link may include, but is not limited to, radio frequency, infrared, laser light, and microwave. The network may, in fact, use a combination of different communications links.
With the exception of a small number of networks that use dedicated communications links between each station, most information networks use a shared transmission medium to carry the transmitted information. Examples of information networks using a shared transmission medium include: Ethernet, token ring, and wireless Ethernet (IEEE 802.11).
However, by sharing a communications medium between multiple stations, there are situations that arise when stations are required to wait a significant amount of time before they are able to transmit their data. Additionally, situations exist when simultaneous transmissions from different stations occur and result in the mutual destruction of the transmissions. Such situations are undesirable in providing quality of service (QoS) to multimedia data transfers and in making efficient use of scarce spectrum on a wireless medium.
For some applications, such as voice telephony, video teleconferencing, and other real-time, bi-directional, interactive applications, extended transfer times can severely and rapidly degrade the performance of the applications to a level that is unacceptable. For example, in voice telephony applications, if the delay between one user speaking and another user listening is greater than a few milliseconds, the delay becomes noticeable to the users and the users' satisfaction level for the telephone connection begins to drop.
One way to ensure that applications requiring a low maximum network latency receive the level of service that they require is to implement some form of QoS transfers of data traffic between stations. In many networks with QoS transfers, communications traffic in a network are partitioned into multiple categories and the categories are parameterized or prioritized according to their specific performance requirements. For example, traffic carrying a telephone conversation between two users will be given a higher priority than traffic carrying data for a file transfer between two computers. By creating categories for the traffic, parameterizing and prioritizing the different categories and ensuring that traffic of higher QoS demands or higher priority receives better service, these networks offer and meet performance guarantees.
In a network that uses a shared communications medium, one commonly used technique to ensure a minimum network performance level is to have a centralized controller controlling access to the communications medium instead of simply relying on a distributed algorithm or random chance to provide access control. At an interval of adjustable duration, the centralized controller polls the stations with data to transmit and grants each one of then a specified duration on the communications medium during which they are free to transmit their data without fear of collisions. Each polled station is then guaranteed to have time on the medium in line with its QoS expectations. This method is sometimes referred to as contention-free communications.
The opposite of contention-free communications is communications with contention, or contention communications. During contention communications, each station with data to transmit must contend with other stations for access to the communications medium. Algorithms, from simple to complex, arbitrate access to the communications medium. However, since the algorithms are non-deterministic and are usually based on a first-come-first-served paradigm, the wait that the stations must endure cannot be predicted, nor is the rate at which the stations can transmit their data. Therefore, communications by distributed contention cannot be used to implement QoS transfers. This is due to the fact that contention communications generally results in communications with low throughput and large delay and jitter.
In the IEEE 802.11 wireless local area network (LAN) technical specifications, provisions for both contention-free and contention communications have been provided in two separate communications periods. Each beacon interval is partitioned into a contention-free period (CFP) and a contention period (CP) for contention-free and contention communications, respectively, with frame exchanges between stations in the two periods employing different access rules and frame formats. As a result, QoS traffic transfer is complex to implement, channel throughput efficiency is relatively low, and co-channel interference mitigation and bandwidth sharing are not straightforward. Furthermore, in IEEE 802.11, the CFP is an option and most implementations of IEEE 802.11 wireless LANs do not support contention-free communications.
A need has therefore arisen for a methodology for providing hybrid contention-free and contention communications during a CP over a shared communications medium on a demand driven basis.